Fix BlurTests assumptions about insets

It is not enough to inset the window frame and assume that leaves the
window contents centred. A device can have different insets on different
sides and for this screenshot test we must keep track of where the
actual centre of the window is.

Instead of insetting the window bounds, leave the window bounds at their
full extents but only check pixels inside the insetted window bounds.

Test: atest CtsWindowManagerDeviceAnimations
Bug: 347853456
Flag: EXEMPT bugfix to test
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:575eb58964ae23dda226aa45bbd44aab24de6016)
Merged-In: I9dc9d00dfde610da34ef38be0ab49beb5db2b266
Change-Id: I9dc9d00dfde610da34ef38be0ab49beb5db2b266